i was going to change my gears to 3.55 and wanted to get some questions answered first what is the stock gear ratio on a 95 ford thunderbird 4.6l v8 and second i heard from many people that it affect your top end speed, is this true and if so by how much, and what else should i expect from changing my gears?

Yes it will effect your top speed. A tune can correct that. The stock ratio is generally 3.08 but 3.27's did exist. I would suggest going at least 3.73 unless you have a supercharger or something like that. The stock drivesahft is very weak I suggest replacing it as well with a better unit.
Alan

is it true i should do anything with the tranny when upgrade the gears, someone told me that i should get a shifting kit which i guess they mean a jmod, but i was wondering because i already have a hypertech chip in which helps the tranny shift better, do i really need to do anything else(except a tranny cooler i plan on installing that)

For a 95 I'd jmod it and refill with Mercon V. I would also upgrade to the 96+ deep pan. If it's the stock trans. then I would also be saving for a 99+ trans. and different torque converter. It's only a matter of time for that year of trans.

Just my .02

Jay

These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.

i'm thinking of doing the same with a posi.how much will the MPG suffer at hiway speeds going to the 3:73 vs 3:55 vs 3:08 some people say its a lot other say it shift to overdrive sooner and you really don't loose much in MPH? Do i need to change the speedometer gears to get it to read right. I don't want a chip (yet!!)

We recently had a meet in Arizona. It was about 100 miles away. One of the people that attended and drove 100 miles get there had 3.73 recently installed. He was turning around 2700 rpm at around 75 mph. With my 3.27s I am turning around 2200. Not a big difference, but try driving around with OD off at 75, that might be around 2700 RPM. I wouldn't want to do it very often for very long. My Cougars are city cars, no reason to take long trips (2000 Grand Marquis with 2.73 is the vacation car), I want to do 3.73s in the 97, half of my daily drive is in the 50-65 mph range, so I think 3.73 would be ideal. With the tune I have now running premium, I averaged 28 mpg, with 200 miles being freeway miles, and 100 miles being short bursts and around town driving. With strickly city stop and go driving, I am getting around 18 (this is all with 3.27).

I don't know what gears my 89 has, it never really mattered because I just drive it and never thought about doing anything because it is a 3.8 car, but I think it was turning closer to 2400 rpm at 75.

You would need to correct for the speedometer, if going from 3.27 to 3.73 most say you don't need a tune to fix that (too much difference and you will bounce it off the rev limiter because of shift duration I think).